1. Field of the Invention
The present invention relates to a control system for a variable-cylinder internal combustion engine which is operated by switching between all-cylinder operation for operating all of a plurality of cylinders and partial-cylinder operation for deactivating operation of some of the plurality of cylinders.
2. Description of the Related Art
Conventionally, a control system of this kind has been proposed in Japanese Patent Application No. 2003-129264 by the present assignee. An internal combustion engine to which is applied the control system is a variable-cylinder internal combustion engine installed on a vehicle, which the control system puts in partial-cylinder operation so as to improve fuel economy, etc. when the opening of an accelerator pedal (stepped-on amount thereof) is smaller than a predetermined reference value, and otherwise, in all-cylinder operation.
As described above, when load on the engine represented by the opening of the accelerator pedal becomes larger than a predetermined reference value, the conventional control system switches the operation of the engine from the partial-cylinder operation to the all-cylinder operation. Therefore, for example, when the reference value is set to a value equivalent to an upper limit of load (hereinafter referred to as “the upper limit load”) corresponding to maximum torque which can be outputted in the partial-cylinder operation, the partial-cylinder operation is performed as long as possible to thereby attain excellent fuel economy. Further, during the partial-cylinder operation, torque variation is more liable to occur than during the all-cylinder operation, since the number of operated cylinders is smaller. Particularly when the load on the engine comes closer to the upper limit load, torque variation increase to increase the vibration and noise of the engine. Thus, when the reference value is set to the upper limit load, very large vibration and noise can be caused.
To eliminate the above inconveniences, it may be considered that the reference value is set to such a smaller value as can suppress torque variation. In this case, however, a region for executing the partial-cylinder operation is limited to shorten time periods over which the partial-cylinder operation execution is executed, which results in degraded fuel economy.